To trap particulate matter contained in the exhaust gas of a diesel engine, a diesel particulate filter (abbreviated to DPF hereafter) provided in an exhaust passage of a diesel engine causes the exhaust gas to pass through porous filter material such that the particulate matter is deposited on a wall surface of hole portions of the filter material. As the deposit amount increases, the exhaust gas flow resistance of the DPF rises and the particulate matter trapping capability of the DPF decreases. When the particulate matter deposit amount reaches a predetermined amount, the temperature of the exhaust gas is raised to bum the deposited particulate matter in the DPF and thereby remove the particulate matter from the DPF. This process is generally referred to as DPF regeneration.
The amount of deposited particulate matter in the DPF can be detected from the differential pressure of the exhaust gas upstream and downstream of the DPF. However, the differential pressure varies according to the flow rate of the exhaust gas passing through the DPF, and the detection precision of a differential pressure sensor which detects the differential pressure may also vary under the influence of variation in the exhaust gas flow rate.